Air electrode material, air electrode, metal-air battery, and fuel cell

ABSTRACT

An air electrode material according to the present disclosure contains a plurality of composite particles, wherein each of the composite particles contains a core particle and a plurality of covering particles covering the core particle, the core particle is formed of a material with catalytic activity for an oxygen reduction reaction, the covering particles are formed of an electrically conductive material and are mechanically bonded to the core particles or other covering particles, and the median size of the core particles ranges from 100 to 1000 times the average primary particle size of the covering particles.

BACKGROUND 1. Field

The present disclosure relates to an air electrode material, an airelectrode, a metal-air battery, and a fuel cell.

2. Description of the Related Art

The following oxygen reduction reaction (electrode reaction) (1) or (2)occurs in an air electrode of a metal-air battery or an air electrode ofa polymer electrolyte fuel cell.O₂+2H₂O+4e ⁻→4OH⁻  (1)O₂+4H⁺+4e ⁻→H₂O  (2)

These oxygen reduction reactions occur at a three-phase interfacebetween gas (O₂), liquid (H₂O), and solid (catalyst). Because theseoxygen reduction reactions are electrochemical reactions, the airelectrodes should have high electric conductivity.

Known air electrodes are produced by kneading a catalyst, anelectrically conductive agent, and a binder. However, the dispersion ofthese constituent materials is difficult to control. Thus, an efficientelectrically conductive path is difficult to form, and a high dischargecurrent density results in a low discharge voltage. Thus, in order toimprove the electric conductivity of an air electrode, a carbon powderloaded with a noble metal or a catalyst powder covered with a carbonfilm is used as a material for air electrodes (see Japanese UnexaminedPatent Application Publication No. 2012-74234, for example).

Meanwhile, there is a powder processing apparatus for forming compositeparticles containing approximately 15-nm titanium oxide mechanicallybonded to the surface of approximately 30-μm silica sand (see JapaneseUnexamined Patent Application Publication No. 2005-270955, for example).

The use of a carbon powder loaded with a noble metal in the productionof an air electrode increases production costs due to the expensivenoble metal. When an air electrode is produced from a catalyst powdercovered with a carbon film, the catalyst particles are covered with thecarbon film, and a gas-liquid-solid three-phase interface needed for anelectrode reaction in the air electrode cannot be stably formed. Thisresults in variations in the electrical characteristics of metal-airbatteries and fuel cells.

SUMMARY

In view of such situations, the present disclosure provides an airelectrode material from which an air electrode catalyst layer with a lowvolume resistivity can be formed at low production costs.

The present disclosure provides an air electrode material that containsa plurality of composite particles, wherein each of the compositeparticles contains a core particle and a plurality of covering particlescovering the core particle, the core particle is formed of a materialwith catalytic activity for an oxygen reduction reaction, the coveringparticles are formed of an electrically conductive material and aremechanically bonded to the core particles or other covering particles,and the median size of the core particles ranges from 100 to 1000 timesthe average primary particle size of the covering particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an air electrode materialaccording to an embodiment of the present disclosure;

FIG. 2 is a schematic enlarged view of a composite particle in an airelectrode material according to an embodiment of the present disclosure;

FIG. 3 is a schematic cross-sectional view of a catalyst layer in an airelectrode according to an embodiment of the present disclosure;

FIG. 4 is a schematic cross-sectional view of a metal-air batteryaccording to an embodiment of the present disclosure; and

FIG. 5 is a schematic cross-sectional view of a fuel cell according toan embodiment of the present disclosure.

DESCRIPTION OF THE EMBODIMENTS

An air electrode material according to an embodiment of the presentdisclosure contains a plurality of composite particles, wherein each ofthe composite particles contains a core particle and a plurality ofcovering particles covering the core particle, the core particle isformed of a material with catalytic activity for an oxygen reductionreaction, the covering particles are formed of an electricallyconductive material and are mechanically bonded to the core particles orother covering particles, and the median size of the core particlesranges from 100 to 1000 times the average primary particle size of thecovering particles.

In the air electrode material, the core particles of the compositeparticles may have a median size in the range of 3 to 100 μm, and thecovering particles may have an average primary particle size in therange of 10 to 100 nm. Thus, the core particles and the coveringparticles have appropriately different particle sizes, and the coveringparticles can be bonded to the core particles or other coveringparticles with high bond strength. Thus, a catalyst layer with a lowvolume resistivity can be formed.

The material of the core particles in the composite particles may be ametal oxide or silver, and the material of the covering particles in thecomposite particles may be a carbon material. This allows a three-phaseinterface for an oxygen reduction reaction to be formed on the surfaceof each core particle and allows electrons to be supplied to thethree-phase interface via the covering particles.

The covering particles in the composite particles may include aplurality of first covering particles mechanically bonded to the surfaceof each core particle, and a plurality of second covering particlesmechanically bonded to other covering particles. This allows the firstcovering particles and the second covering particles to form anelectrically conductive path. Thus, a stable electrically conductivepath can be formed. This can also reduce the decrease in dischargevoltage even in the case of electrical discharge at a high electriccurrent density.

The covering particles in the composite particles may adhere to the coreparticles with such strength that 80% or more of the covering particlesare not detached from the composite particles after ultrasonication ofthe composite particles at 20 kHz for 3 minutes. This allows the coreparticles and the covering particles to be combined and allows thecovering particles to form a stable electrically conductive path.

The present disclosure also provides an air electrode containing acatalyst layer. The catalyst layer contains an air electrode materialaccording to the present disclosure and has a volume resistivity of 0.65Ω·cm or less. Due to the low volume resistivity of the catalyst layer,the air electrode can have low internal resistance.

The present disclosure also provides a metal-air battery that includesan air electrode according to the present disclosure, a metal electrode,and an electrolyte. Due to the low internal resistance of the airelectrode, a metal-air battery according to the present disclosure canhave good discharging characteristics.

The present disclosure also provides a fuel cell that includes an airelectrode according to the present disclosure, a fuel electrode, and anelectrolyte. Due to the low internal resistance of the air electrode, afuel cell according to the present disclosure can have good dischargingcharacteristics.

Some embodiments of the present disclosure will be described below withreference to the drawings. These embodiments illustrated in the drawingsand the following description are only examples, and the scope of thepresent disclosure is not limited to these embodiments.

First Embodiment (Air Electrode Material)

FIG. 1 is a schematic cross-sectional view of an air electrode materialaccording to the present embodiment. FIG. 2 is a schematic enlarged viewof a composite particle in an air electrode material according to thepresent embodiment.

An air electrode material 5 according to the present embodiment containscomposite particles 2. Each of the composite particles 2 contains a coreparticle 3 and covering particles 4 covering the core particle 3. Thecore particles 3 are formed of a material with catalytic activity for anoxygen reduction reaction. The covering particles 4 are formed of anelectrically conductive material and are mechanically bonded to the coreparticles 3 or other covering particles 4. The median size of the coreparticles 3 ranges from 100 to 1000 times the average primary particlesize of the covering particles 4.

The air electrode material 5 according to the present embodiment may bea powder, may be one contained in an air electrode of a metal-airbattery, or may be one contained in an air electrode of a fuel cell.

Each of the composite particles 2 contains the core particle 3 and thecovering particles 4 covering the core particle 3. More specifically, inthe composite particles 2, the covering particles 4 are fixed to thesurface of the core particles 3. The composite particles 2 can be formedby combining the core particles 3 and the covering particles 4 by amechanochemical method.

The core particles 3 are formed of a material with catalytic activityfor an oxygen reduction reaction. The material of the core particles 3is a metal oxide or silver, for example. More specifically, the materialof the core particles 3 may be a manganese oxide, such as MnO₂ or Mn₃O₄,Ag, or a perovskite metal oxide. The core particles 3 of the compositeparticles 2 allow an electrode reaction of the air electrode (an oxygenreduction reaction) to occur on the surface of the core particles 3.

The core particles 3 may have a median size in the range of 3 to 100 μm,preferably 3 to 50 μm.

The median size D₅₀ of the core particles 3 can be calculated from theparticle size distribution of the core particles 3 contained in the airelectrode material 5.

The covering particles 4 are formed of an electrically conductivematerial. The covering particles 4 cover the surface of the coreparticles 3. The covering particles 4 can form an electricallyconductive path and can promptly supply electrons to the surface of thecore particles 3, on which an electrode reaction occurs. The coveringparticles 4 can also reduce the decrease in discharge voltage even inthe case of electrical discharge at a high electric current density.

The covering particles 4 can cover the core particles 3 such thatadjacent two covering particles 4 are in contact with each other. Alayer or layers of the covering particles 4 may cover each of the coreparticles 3. A porous layer of the covering particles 4 may cover eachof the core particles 3. The covering particles 4 may substantiallyentirely cover the core particles 3.

The covering particles 4 may be electrically conductive carbonparticulates. More specifically, the material of the covering particles4 may be carbon black, carbon fiber, carbon nanotube, activated carbon,or graphite.

The covering particles 4 may have an average primary particle size inthe range of 10 to 100 nm, preferably 10 to 50 nm.

The average primary particle size of the covering particles 4 may be thearithmetic mean size determined by electron microscopic observation.

Each of the composite particles 2 may contain the core particle 3,covering particles 4 a mechanically bonded to the surface of the coreparticle 3, and covering particles 4 b mechanically bonded to thecovering particles 4 a.

The covering particles 4 a are mechanically bonded to the core particles3. The covering particles 4 a may be mechanically directly bonded to thecore particles 3. For example, the covering particles 4 a are partlyburied in and engage with the core particles 3 and are thereby bonded tothe core particles 3. For example, the covering particles 4 a are partlyplaced in recessed portions of the core particles 3 and engage with thecore particles 3 and are thereby bonded to the core particles 3. Forexample, the covering particles 4 a may be partly buried in the coreparticles 3, as illustrated in FIG. 2.

The covering particles 4 a are mechanically bonded to and thereby fixedto the surface of the core particles 3, and can form a stableelectrically conductive path. The covering particles 4 a can be fixed tothe core particles 3 without an adhesive and can therefore form a space6 between adjacent two covering particles 4 a or between the coveringparticles 4 a and the core particles 3. A gas or liquid can enter thespace 6. Thus, a gas or liquid can come into contact with the coreparticles 3 and can stably form a three-phase interface for an electrodereaction. For example, as illustrated in FIG. 2, a gas or liquid canenter a space 6 between adjacent two covering particles 4 a and the coreparticle 3.

The covering particles 4 b may be mechanically directly bonded to thecovering particles 4 a.

For example, the covering particles 4 b are partly buried in and engagemechanically with the covering particles 4 a and are thereby bonded tothe covering particles 4 a. For example, the covering particles 4 b arepartly placed between adjacent two covering particles 4 a, engagemechanically with the covering particles 4 a, and are thereby bonded tothe covering particles 4 a. For example, the covering particles 4 a maybe bonded to the covering particles 4 b, as illustrated in FIG. 2.

Since the composite particles 2 contain the covering particles 4 b, boththe covering particles 4 a and 4 b can form an electrically conductivepath. Thus, a more stable electrically conductive path can be formed.This can also reduce the decrease in discharge voltage even in the caseof electrical discharge at a high electric current density.

The core particle 3 may be covered with 2 to 10 layers of the coveringparticles 4.

The covering particles 4 in the composite particles 2 may adhere to thecore particles 3 with such strength (adhesion strength) that 80% or moreof the covering particles 4 are not detached from the compositeparticles 2 after ultrasonication of the composite particles 2 at 20 kHzfor 3 minutes. This allows the core particles 3 and the coveringparticles 4 to be combined and allows the covering particles 4 to form astable electrically conductive path. Ultrasonication can be performed onan aqueous suspension of the composite particles 2. The proportion ofthe covering particles 4 not detached from the composite particles 2after ultrasonication of the composite particles 2 at 20 kHz for 3minutes is referred to as adhesion strength. For example, the adhesionstrength is 80% or more, preferably 85% or more.

The median size of the core particles 3 ranges from 100 to 1000 timesthe average primary particle size of the covering particles 4. Thus, thecore particles 3 and the covering particles 4 have appropriatelydifferent particle sizes, and the covering particles 4 can be bonded tothe core particles 3 with high bond strength. This was proved in theexperiment described later. If the median size of the core particles 3ranges from 100 to 1000 times the average primary particle size of thecovering particles 4, the composite particles 2 can form a catalystlayer with a low volume resistivity, and a metal-air battery with a highdischarge voltage can be produced. This was proved in the experimentdescribed later.

The air electrode material 5 containing the composite particles 2 can beproduced by applying impact force, compression force, and/or shear forceto a mixture of a raw powder of the core particles 3 and a raw powder ofthe covering particles 4 in a particle composing machine. Applyingimpact force, compression force, and/or shear force to the mixed powdermechanically binds the covering particles 4 to the core particles 3,thus forming the composite particles 2.

The median size of the raw powder of the core particles 3 can beconsidered to be the median size of the core particles 3 in thecomposite particles 2. The average primary particle size of the rawpowder of the covering particles 4 can be considered to be the averageprimary particle size of the covering particles 4 in the compositeparticles 2.

Second Embodiment (Air Electrode)

FIG. 3 is a schematic cross-sectional view of a catalyst layer 10 in anair electrode 11 according to the present embodiment.

The air electrode 11 according to the present embodiment includes thecatalyst layer 10, which contains the air electrode material 5 accordingto the first embodiment. The catalyst layer 10 has a volume resistivityof 0.65 Ω·cm or less, preferably 0.6 Ω·cm or less.

The air electrode 11 according to the present embodiment employs anoxygen gas as an electrode active material and contains a catalyst foran oxygen reduction reaction. The air electrode 11 may be an airelectrode of a metal-air battery or an air electrode of a fuel cell.

The air electrode 11 according to the present embodiment can include acurrent collector. The air electrode 11 according to the presentembodiment can include a water-repellent film.

The catalyst layer 10 contains the composite particles 2. Each of thecomposite particles 2 contains the core particle 3 and the coveringparticles 4 covering the core particle 3. The core particles 3 areformed of a material with catalytic activity for an oxygen reductionreaction. The covering particles 4 are formed of an electricallyconductive material and are mechanically bonded to the core particles 3or other covering particles 4. The median size of the core particles 3ranges from 100 to 1000 times the average primary particle size of thecovering particles 4.

The catalyst layer 10 can contain an electrically conductive agent 8.The electrically conductive agent 8 can improve the electricalconductivity of the catalyst layer 10.

The electrically conductive agent 8 may be electrically conductivecarbon particulates. More specifically, the material of the electricallyconductive agent 8 may be carbon black, carbon fiber, carbon nanotube,activated carbon, or graphite. The electrically conductive agent 8 mayhave an average primary particle size in the range of 10 to 100 nm,preferably 10 to 50 nm. The electrically conductive agent 8 may be theraw powder of the covering particles 4.

The catalyst layer 10 can contain a binder 9. The binder 9 can bind thecomposite particles 2 and the electrically conductive agent 8 in thecatalyst layer 10 and stabilize the characteristics of the catalystlayer 10. The binder 9 may suitably be a fluoropolymer with high alkaliresistance. The binder 9 may be polytetrafluoroethylene (PTFE), whichgrows in a fibrous form, binds particles together, has high waterrepellency, and is resistant to heat.

The catalyst layer 10 can be formed by kneading the air electrodematerial 5 according to the first embodiment, the electricallyconductive agent 8, the binder 9, and a solvent and by shaping themixture into a film.

The air electrode 11 can be produced by pressing the catalyst layer 10and a current collector.

Third Embodiment (Metal-Air Battery)

FIG. 4 is a schematic cross-sectional view of a metal-air batteryaccording to the present embodiment.

A metal-air battery 18 according to the present embodiment includes theair electrode 11 according to the second embodiment, a metal electrode15, and an electrolyte 16.

The metal-air battery 18 according to the present embodiment includesthe metal electrode 15 as a negative electrode (anode) and the airelectrode 11 as a positive electrode (cathode). For example, themetal-air battery 18 is a zinc-air battery, a lithium-air battery, asodium-air battery, a calcium-air battery, a magnesium-air battery, analuminum-air battery, or an iron-air battery.

For zinc-air batteries, the metal electrode 15 may be formed of metalzinc, and the electrolyte 16 may be aqueous potassium hydroxide.

Fourth Embodiment (Fuel Cell)

FIG. 5 is a schematic cross-sectional view of a fuel cell 35 accordingto the present embodiment.

The fuel cell 35 according to the present embodiment includes the airelectrode 11 according to the second embodiment, a fuel electrode 29,and an electrolyte 32.

The fuel cell 35 according to the present embodiment may include acation-exchange membrane or an anion-exchange membrane as a polymerelectrolyte membrane.

Experiments

<Preparation of Composite Particles>

A MnO₂ powder (trade name: CMD-K200, manufactured by Chuo Denki KogyoCo., Ltd., median size: 2.5, 3.5, 9.0, 30, or 35 μm) serving as coreparticles and carbon black (average primary particle size: 19, 30, 48,or 85 nm) serving as covering particles were mixed at a weight ratio ofMnO₂ powder:carbon black=100:3 to prepare mixed powders according toExamples 1 to 6 and Comparative Examples 1 and 2. The mixed powders haddifferent ratios of the median size of the MnO₂ particles to the averageprimary particle size of the covering particles. The carbon black was“Denka Black” manufactured by Denka Company Limited or “Asahi #50HG” or“Asahi AX-015” manufactured by Asahi Carbon Co., Ltd.

10 g of each of the mixed powders according to Examples 1 to 6 andComparative Examples 1 and 2 was subjected to impact force, compressionforce, and/or shear force in a particle composing machine (trade name:Nobilta, manufactured by Hosokawa Micron Corporation) at the number ofrevolutions and for the processing time listed in Table 1. Thus, powdersof composite particles according to Examples 1 to 6 and ComparativeExamples 1 and 2 were prepared. The covering particles (carbon black)were mechanically bonded to the surface of the core particles (MnO₂particles) Table 2 lists the ratio of the median size D_(cat) of thecore particles to the average primary particle size D_(cb) of thecovering particles in the powders of composite particles according toExamples 1 to 6 and Comparative Examples 1 and 2.

TABLE 1 Amount of charged Number of revolutions in Processing time inmaterial Nobilta [rpm] Nobilta [min] 10 g 3500 10

TABLE 2 Average Median primary Adhesion Volume Discharge size ofparticle strength resistivity voltage core size of of of of particlescovering covering catalyst zinc-air D_(cat) particles D_(cat)/ particleslayer battery [μm] D_(cb) [nm] D_(cb) [%] [Ω · cm] [V] Example 1 3.5 30117 93 0.52 1.25 Example 2 9.0 30 300 95 0.45 1.23 Example 3 30 30 100088 0.55 1.20 Example 4 45 48 938 86 0.58 1.20 Example 5 70 85 815 820.62 1.17 Example 6 3.5 19 184 89 0.54 1.21 Compar- 2.5 30 80 52 0.901.01 ative example 1 Compar- 35 30 1200 68 0.71 1.12 ative example 2<Evaluation of Adhesion Strength of Covering Particles to CoreParticles>

Each of the powders of composite particles according to Examples 1 to 6and Comparative Examples 1 and 2 was dispersed in water containing 2% byweight of a surfactant to prepare a suspension with a solid content of5% by weight. The suspension was subjected to ultrasonication for 3minutes with an ultrasonic homogenizer (output: 20 kHz).

The suspension subjected to ultrasonication was filtered through afilter paper through which the carbon black particles can pass but thecomposite particles cannot pass. Thus, a residue containing thecomposite particles was separated from the filtrate containing thecarbon black particles detached from the composite particles.

The residue was dried and pressed into a pellet. The pellet of thesample subjected to ultrasonication was subjected to a fluorescent X-rayelemental analysis. An unprocessed sample prepared from the compositeparticles not subjected to ultrasonication was also subjected to thefluorescent X-ray elemental analysis to compare the carbon componentcontents. The proportion of carbon black particles not detached from thecomposite particles by ultrasonication was calculated as “adhesionstrength” from the ratio of (the carbon content of the sample subjectedto ultrasonication)/(the carbon content of the unprocessed sample).Table 2 lists the adhesion strength of Examples 1 to 6 and ComparativeExamples 1 and 2.

The powders of composite particles according to Examples 1 to 6 had anadhesion strength of 80% or more, showing that the covering particleswere negligibly detached from the core particles by ultrasonication.This shows that the covering particles adhered strongly to the coreparticles in the powders of composite particles according to Examples 1to 6.

In contrast, the powders of composite particles according to ComparativeExamples 1 and 2 had an adhesion strength of 70% or less, showing thatmany covering particles were detached from the core particles byultrasonication.

These results show that the covering particles adhered strongly to thecore particles in the powders of composite particles in which the mediansize D_(cat) of the core particles ranged from 100 to 1000 times theaverage primary particle size D_(cb) of the covering particles.

<Formation of Air Electrode Catalyst Layer>

Carbon black (trade name: Denka Black, manufactured by Denka CompanyLimited, average primary particle size: 30 nm) was added to the powdersof composite particles according to Examples 1 to 6 and ComparativeExamples 1 and 2 to prepare mixed powders. The amount of the carbonblack was 1.5 times the weight of the powders. The carbon black was anelectrically conductive agent. Each of the mixed powders was mixed witha binder (PTFE dispersion liquid “D-210C”, manufactured by DaikinIndustries, Ltd., solvent: water, solid content: 60% by weight) in aplanetary mixer to prepare a mixture A. The weight of the binder was 25%of the total solids. The total solid content was adjusted to be 50% byweight with water. The mixture A was kneaded in a mortar to prepare alump of mixture B. The mixture B was formed into a sheet with a rollingmill to form air electrode catalyst layers according to Examples 1 to 6and Comparative Examples 1 and 2. The air electrode catalyst layers hada thickness in the range of 100 μm to 2 mm.

Air electrode catalyst layers according to Comparative Examples 3 to 7were formed from a MnO₂ powder (trade name: CMD-K200, manufactured byChuo Denki Kogyo Co., Ltd., median size: 2.5, 3.5, 9.0, 30, or 35 μm)instead of the powders of composite particles. The other formingconditions were the same as in Examples 1 to 6 and Comparative Examples1 and 2. Table 3 lists the ratio of the median size of each of the MnO₂powders according to Comparative Examples 3 to 7 to the average primaryparticle size of the electrically conductive agent (carbon black).

TABLE 3 Average primary Median size particle size of Volume Discharge ofMnO₂ electrically resistivity of voltage of particles D_(cat) conductiveagent catalyst zinc-air [μm] D_(cb) [nm] D_(cat)/D_(cb) layer [Ω · cm]battery [V] Comparative 9.0 30 300 0.91 1.08 example 3 Comparative 3.530 100 0.93 1.01 example 4 Comparative 30 30 1000 0.70 1.02 example 5Comparative 2.5 30 80 0.93 0.91 example 6 Comparative 35 30 1200 0.730.96 example 7<Evaluation of Electrical Conductivity of Air Electrode Catalyst Layer>

The volume resistivity of the air electrode catalyst layers according toExamples 1 to 6 and Comparative Examples 1 to 7 was measured with aresistivity meter (Loresta GX MCP-T700 manufactured by MitsubishiChemical Analytech Co., Ltd.). Tables 2 and 3 show the results.

The air electrode catalyst layers formed from the powders of compositeparticles with high adhesion strength according to Examples 1 to 6 had avolume resistivity of 0.65 Ω·cm or less. In contrast, the air electrodecatalyst layers formed from the powders of composite particles with lowadhesion strength according to Comparative Examples 1 and 2 had a volumeresistivity of 0.7 Ω·cm or more.

The air electrode catalyst layers according to Comparative Examples 3 to7, in which the MnO₂ powder and carbon black were not used incombination, had a volume resistivity of 0.7 Ω·cm or more.

These results show that the air electrode catalyst layers formed fromthe powders of composite particles in which the median size D_(cat) ofthe core particles ranged from 100 to 1000 times the average primaryparticle size D_(cb) of the covering particles had a low volumeresistivity.

It was also shown that the adhesion strength of the covering particlesto the core particles correlates with the electrical conductivity of theair electrode catalyst layers. This proved that adhesion strength is anappropriate indicator of the “result” of the combination of particles.

The air electrode catalyst layers formed from the powders of compositeparticles according to Examples 1 to 6 had a lower volume resistivity.This shows that the constituent materials of the air electrode catalystlayers had improved dispersion uniformity and that the combination ofparticles is effective in uniformly dispersing materials with verydifferent particle sizes or specific gravities.

<Production of Air Electrode>

Each of the air electrode catalyst layers according to Examples 1 to 6and Comparative Examples 1 to 7 was placed on a PTFE surface of awater-repellent film (“Temish” manufactured by Nitto Denko Corporation,front side: PTFE, backing: PP). A current collector (a Ni meshmanufactured by Nilaco Corporation, mesh opening: #20) was placed on theair electrode catalyst layer. The laminate was pressed at normaltemperature at 2.15 kN/cm² for 2 minutes to form air electrodesaccording to Examples 1 to 6 and Comparative Examples 1 to 7.

<Production of Zinc-Air Battery>

Zinc-air batteries including air electrodes according to Examples 1 to 6and Comparative Examples 1 to 7 were produced, as illustrated in FIG. 4.The electrolyte was aqueous 7 M KOH, and the metal electrode was a zincplate.

<Evaluation of I-V Characteristics>

The I-V characteristics of the zinc-air batteries according to Examples1 to 6 and Comparative Examples 1 to 7 were evaluated with a batterytester (battery test system PFX2011 manufactured by Kikusui ElectronicsCorporation). Tables 2 and 3 list the discharge voltage of each batteryat an electric current density of 30 mA/cm².

The zinc-air batteries according to Examples 1 to 6 had a dischargevoltage of 1.15 V or more, whereas the zinc-air batteries according toComparative Examples 1 to 7 had a discharge voltage of 1.12 V or less.

These results show that the zinc-air batteries had a higher dischargevoltage when the air electrode catalyst layers were formed from thepowders of composite particles in which the median size D_(cat) of thecore particles ranged from 100 to 1000 times the average primaryparticle size D_(cb) of the covering particles.

It was also shown that the adhesion strength of the covering particlesto the core particles correlates with the discharging characteristics ofthe metal-air batteries. This proved that adhesion strength is anappropriate indicator of the “result” of the combination of particles.

The present disclosure contains subject matter related to that disclosedin Japanese Priority Patent Application JP 2016-143465 filed in theJapan Patent Office on Jul. 21, 2016, the entire contents of which arehereby incorporated by reference.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. An air electrode material comprising: a pluralityof composite particles, wherein each of the composite particles containsa core particle and a plurality of covering particles covering the coreparticle, the core particle is formed of a material with catalyticactivity for an oxygen reduction reaction, the covering particles areformed of an electrically conductive material and are mechanicallybonded to the core particles or other covering particles, and a mediansize of the core particles ranges from 100 to 1000 times an averageprimary particle size of the covering particles.
 2. The air electrodematerial according to claim 1, wherein the core particles have a mediansize in the range of 3 to 100 μm, and the covering particles have anaverage primary particle size in the range of 10 to 100 nm.
 3. The airelectrode material according to claim 1, wherein the material of thecore particle is a metal oxide or silver, and the material of thecovering particles is a carbon material.
 4. The air electrode materialaccording to claim 1, wherein the plurality of covering particlesinclude a plurality of first covering particles mechanically bonded to asurface of the core particle, and a plurality of second coveringparticles mechanically bonded to other covering particles.
 5. The airelectrode material according to claim 1, wherein the covering particlesadhere to the core particles with such strength that 80% or more of thecovering particles are not detached from the composite particles afterultrasonication of the composite particles at 20 kHz for 3 minutes. 6.An air electrode comprising a catalyst layer containing the airelectrode material according to claim 1, wherein the catalyst layer hasa volume resistivity of 0.65 Ω·cm or less.
 7. An metal-air batterycomprising: the air electrode according to claim 6; a metal electrode;and an electrolyte.
 8. A fuel cell comprising: the air electrodeaccording to claim 6; a fuel electrode; and an electrolyte.